This invention relates to a Darlington power transistor, and more particularly to a p-n-p type Darlington power transistor.
In case of putting transistors of the same conductivity type into the Darlington connection, their collectors are connected in common. Accordingly, two or more transistors to be Darlington-connected can be fabricated in a semi-conductor substrate which serves as the common collectors. A driving transistor, namely, a transistor at the preceding stage amplifies an input current and drives an output transistor, namely, a transistor at the succeeding stage. Such a multiple transistor drives a load satisfactorily with a comparatively small input current. In this invention, the multiple transistor as above described shall be termed the "Darlington power transistor".
In the Darlington power transistor, a base region of the driving transistor and a base region of the output transistor are formed in the semiconductor substrate as the common collectors, and the emitter of the driving transistor and the base of the output transistor are connected by an interconnection or the like.
Breeder resistances are connected to the respective bases of the driving transistor and the output transistor in order that even when an undersirable leakage current has arisen in the transistor of the preceding stage the transistor of the succeeding stage may not respond to the leakage current and that the operation may be stabilized.
The breeder resistance for the driving transistor can be constructed by, for example, a semiconductor resistance region which is made simultaneously with the base regions of the driving transistor and the output transistor and which is elongate in a manner to continually extend between the base regions. For the breeder resistance for the output transistor, the spreading resistance of the base region can be utilized by short-circuiting a part of the base-emitter junction of the output transistor by means of an emitter electrode.
In order to stabilize electrical characteristics, the surfaces of the base, emitter and collector regions of the Darlington power transistor are covered with a protective film such as silicon oxide film.
The Darlington power transistor has been expected to execute a stable and favorable operation owing to the adoption of the breeder resistances as stated above.
According to experiments, however, the operation was unstable particularly in a p-n-p type Darlington power transistor, and characteristics such as current gain did not demonstrate satisfactory values.
It has been revealed by the inventor's study that, in spite of the adoption of the breeder resistances as stated previously, channels causing channel currents which are not negligible are formed between the base regions of the output transistor and in the surface of the semiconductor substrate in the vicinity of the semiconductor resistance region.